Conventionally, in connection with an assembly product of, for example, an electric device, an electronic device and the like, what is well known is a structure in which, in the case where a rotary element, e.g., a gear or a pulley, is to be rotatably assembled to an assembly pedestal, a hollow shaft is provided to stand upright on a pedestal, and the rotary element is assembled to the tip side of the hollow shaft so as to be rotatable about the hollow shaft. Refer to Patent Literatures 1 or 2, for example.
Further, as to such a structure, in the case where the rotary element is assembled to the tip side of the hollow shaft, the following structure is widely employed: a female screw portion is provided to the tip side of the hollow shaft; a male screw member such as a screw which penetrate through the center portion of the rotary element is screwed and tightened with the female screw portion on the hollow shaft tip side; and in this tightened state, a slight gap is formed between the head portion of the male screw member or a washer and the hollow shaft tip portion. Employing such a structure, it becomes possible to surely prevent the rotary element from coming off from the tip portion of the hollow shaft while rotatably holding the rotary element at the tip portion of the hollow shaft.